19S 
1>ALLIAL FUSION. 
The Pallial Region. 
'File ]\Iantle, or Palliiiiii, is a tliiii and vascular fold of the iiitegu- 
iiieiit, which is fused to and covers the dorsum of the body and arises 
during development around the primitive shell gland, its thickened 
and glandular margins, which are the active formative organs of the 
shell, enclosing, in conjunction with the body wall, a pallial cavity 
communicating more or less freely with the external medium and pro- 
tecting the resiiiratory organs, which may exist as gills or ctenidia 
for the respiration of water or, in the terrestrial species, may take 
the form of a lung constituted by an intricate network of blood vessels 
distributed upon the roof of the pallial cavity. In the Streptoneures 
and the more i)riniitive Pelecypods large I I}qiobranchial mucous 
glands are developed within the pallial cavity between the rectum and 
the branchiic. 
The Fusion of the pallial margins to each other and to the body 
wall to form an enclosed resi)iratory cavity, has not been carried to a 
great extent amongst the Streptoneures of our fauna, the cavity being 
more or less freely open anteriorly to the inhabited medium. 
In the Euthynenres this process has, however, become further 
advanced, and the jiallial margins have fused with the body wall, 
leaving only a .small contractile lateral oribce for respiratory and 
excretory purposes, which ilivides the mantle more or less distinctly 
into an anterior and a posterior lobe (.see p. 144, f. 304). 
In the Pelecyiioda the mantle is e(|ually developed laterally, en- 
closing a symmetrical iiallial cavity. In the more archaic forms, as 
Di.-iSr.nms illu.'itratmg the position anti mode of fusion of the mantle lohes of the Pelecypoda 
(after Lang). The arrow.s indicate the direction of the currents. right mantle iobe \f. foot. 
Fig. Illustrating the primitively open mantle with unseparated respiratory currents. 
Fig. HOI.— liiforate Pelecypod, showing a single point of fusion of the right and left mantle 
lobes, and separating the inhalent and exhalent currents. 
Fig 392.— A Triforate Pelecypod, showing two points of fusion, which form the e.'ihalent and 
inhalent orifices and a large pedal aperture. 
Nucula or Trujonia, the pallial margins are quite free ventrally, no 
provi,sion being made, except sucb as is provided by cilial action, 
tor the separation ot the pure incoming and the impure out-flowing 
currents. Specialization is evidenced by a vascular fusion of the 
mantle margins, primarily to ensure the separation of the in-Howing 
